1. Field of the Invention
The present invention relates to a process for forming a polyimide layer on an underlying metal surface, such as a substrate containing a pattern of metallization, e.g. copper. More particularly, the process involves formation of an intermediate passivation layer, which serves to passivate the metal from interaction with the polyimide precursor material.
2. Description of the Prior Art
Polyimides are being used increasingly in the microelectronics industry. For example, polyimides formed by the thermal curing of polyimide precursors, such as polyamic acids, can be used as passivation dielectrics on semiconductor devices. See, for example, U.S. Pat. No. 4,590,258, issued to Linde, et al., on May 20, 1986.
Two types of interfaces between polyimides and metals have been studied, viz. metal-on-polymer and polymer-on-metal. Most attention has been given to the first type, formed by vapor deposition of metal films on cured polyimide layers. In this type, problems arise due to poor adhesion, leading to oxidation and moisture penetration at the interface. Techniques have been developed in the art in an effort to improve the adherence. In U.S. Pat. No. 4,152,195, issued to Bahrle, et al., on May 1, 1979, the process involves only partially curing the polyimide precursor material before vapor depositing the desired metal, followed by completing the cure of the polyimide.
Also illustrative of the art is U.S. Pat. No. 4,797,307, issued to Kunimoto, et al., on Jan. 10, 1989, which provides a process for preparing a polyimide film having improved adhesiveness to a metal foil, such as copper. According to the process, a treating solution containing an aminosilane is coated on a surface of a polyamic acid film, which is then heated to effect imidization. The cured polyimide film is then pressed together with an adhesive-coated copper foil to produce a laminate sheet.
The second type of interface, i.e. polymer-on-metal, is formed by coating a polyimide precursor material on to a metal film, and then carrying out a curing step. However, the metal, e.g. copper, has been shown to interact with commonly used polyimide precursor materials (polyamic acids). See Kim, et al., "Adhesion And Interface Studies Between Copper And Polyimide", J. Adhesion Sci. Tech., Vol. 1, No. 4 (1987), pp. 331-339. Copper and other reactive metals form salts that retard thermal imidization and decompose the polyimide polymer during the high temperature curing step.
In the field of multilevel interconnection metallurgy, it is known to use a layer of silicon nitride or polyimide as an interlevel dielectric between metal layers or a dual component dielectric that includes a layer of each. See U.S. Pat. No. 4,423,547, issued to Farrar, et al. on Jan. 3, 1984, in which a layer of silicon nitride is deposited on metal, followed by deposition of a thicker layer of polyimide. In such systems, the silicon nitride will serve to passivate the underlying metal; however, it does present a problem, in that the nitride is prone to cracking due to a large stress-mismatch with respect to the polyimide. Further, it tends to lock in moisture, leading to oxidation of the metal, and degrades overall capacitance of the system.
Thus, there is a need for an improved process to passivate the metal, thus reducing or eliminating interaction with the polyimide precursor material, but without significantly degrading the electrical characteristics of the metal.
It is also known to form silsesquioxane polymers for use as insulating layers in semiconductor devices. For example, in Eur. Pat. Appln., published under U.S. Pat. No. 0,226,208 on Jun. 24, 1987, an insulating layer is formed by applying to a substrate a prepolymer, and then heating it at a temperature above 400.degree. C. in the presence of oxygen. The prepolymer is prepared by hydrolyzing and polycondensating a mixture of a tetraalkoxysilane, a trialkoxysilane and a dialkoxysilane in a select mole ratio.
In U.S. Pat. No. 4,626,556, issued to Nozue, et al., on Dec. 2, 1986, water is reacted with a trihalogenosilane in the production of a non-amino-containing silsesquioxane polymer, which is used in a mixture with a compound which generates crosslinking-reaction-active species upon irradiation, in the formation of an insulating layer. In U.S. Pat. No. 4,723,978, issued to Clodgo, et al., on Feb. 9, 1988, an organoglass insulating layer is produced by first forming a modified ladder-type silsesquioxane polymer from a silanol solution, and then treating it in an oxygen plasma.
However, none of these approaches involves the formation of a silsesquioxane polymer as a passivation layer in a metal/polyimide structure.